The present invention relates to a device for exhaust gas recirculation control in an internal combustion engine, and more particularly relates to a device for controlling exhaust gas recirculation, in an internal combustion engine as used in a road vehicle or the like, which matches the ratio of exhaust gas recirculation appropriately to the load to which the engine is subjected.
It is already known, as an example of an exhaust gas recirculation control device for a motor vehicle internal combustion engine, to utilize a back pressure control type exhaust gas recirculation control device, which is provided with: an exhaust gas recirculation control valve, incorporated in the exhaust gas recirculation passage, which responds to an increase in the vacuum which is supplied to its diaphragm chamber by more opening said exhaust gas recirculation passage; a passage means for bringing the vacuum to the diaphragm chamber of the exhaust gas recirculation control valve; an orifice element formed in the exhaust gas recirculation path on the upstream side of the exhaust gas recirculation control valve, so as to define a pressure chamber between itself and the exhaust gas recirculation control valve; and a vacuum control valve mounted on said passage for bringing vacuum. In such an exhaust gas recirculation control device, the exhaust gas recirculation control valve is operated by vacuum which is adjusted, in response to the exhaust gas pressure in the pressure chamber, by the operation of the vacuum control valve. By keeping the pressure in the pressure chamber at a substantially constant value, fairly close to atmospheric pressure, the amount of exhaust gas recirculation, relative to the amount of inlet air, or in other words the exhaust gas recirculation ratio, is held substantially constant while the exhaust gas recirculation is carried out.
In this back pressure control type of exhaust gas recirculation control device, irrespective of the engine load, the exhaust gas recirculation ratio is kept substantially constant. In this case, if the exhaust gas recirculation ratio is low, no particular problems arise; but if the exhaust gas recirculation ratio is made higher in order to make more effective the reduction of NOx emission from the engine, the problem arises that the drivability of the engine is worsened when it is operated under low load conditions, and, depending on the characteristics of the individual engine, the drivability may also worsen when the engine is operated under high load conditions.
The worsening of drivability under conditions of low load is caused by the fact that under conditions of low load the residual amount of exhaust gas remaining in the combustion chambers of the engine increases, and thus the effective exhaust gas recirculation ratio ([recycled exhaust+residual exhaust gas]/air intake amount) becomes too high. The worsening of drivability under conditions of high load is because the reduction in engine output brought about by the recirculation of exhaust gases makes the acceleration of the engine lower.
Therefore, in order to carry out exhaust gas recirculation without worsening the drivability of the vehicle incorporating the internal combustion engine, and still sufficiently to reduce the quantity of NOx emissions to comply with the standards for control of exhaust emission, which are becoming more and more severe nowadays, it is desirable that the exhaust gas recirculation ratio should be matched to the load of the engine in such a manner that the exhaust gas recirculation ratio is lowered in low load operation compared to medium load operation; or, alternatively, that the exhaust gas recirculation ratio is lowered in low and high load operation compared to medium load operation.